The popularity of the internet has fundamentally changed the underlying information communication infrastructure, and has led to the worldwide telecom boom in the late 1990s and early 2000s. The volume of internet traffic continues to grow rapidly fueled by the emergence of new applications, thus increasing the demand for higher bandwidths. The exponential internet traffic growth projections place enormous transmission rate demand on the underlying information infrastructure at every level, from the core to access networks.
In order to satisfy high capacity demands, according to some industry experts, the 1 TbE standard should be completed in foreseeable future. Coherent optical OFDM is one possible pathway towards achieving beyond 1 Tb/s optical transport. Another approach is based on multidimensional coded modulation. Namely, by increasing the number of dimensions (i.e., the number of orthonormal basis functions), we can increase the aggregate data rate of the system without degrading the bit error rate (BER) performance as long as orthogonality among basis functions is preserved. Multidimensional signal constellations for optical communications have been used, but so far have been related to single carrier and SMF-based systems.
From Shannon's theory, information capacity is a logarithmic function of signal-to-noise ratio, but a linear function of the number of dimensions. By increasing the number of dimensions D the spectral efficiency can be dramatically improved. At the same time, in D-dimensional space (D>2) for the same average symbol energy the Euclidean distance between signal constellation points can be increased when compared to the conventional in-phase (I)/quadrature (Q) two-dimensional signal-space. The four-dimensional space, with two coordinates being in-phase/quadrature components in x-polarization (Ix and Qx) and two coordinates being in-phase/quadrature components in y-polarization (Iy and Qy), has already been intensively studied.